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Introduction Description Introduction This bird • has disks of stiff feathers around its eyes that reflect sound waves to its ear openings • must capture the equivalent of 7 to 12 mice a day to meet its food requirements • is active during the day as well as at night, unlike most owls • moves to sit on patches of snow or ice as the ground becomes bare with the approach of spring Description One of the heaviest of North American owls, the Snowy Owl Bubo scandiacus stands nearly half a metre tall, with a wingspan of almost 1.5 m. As is the case with most diurnal birds of prey—those that are active during the day—the female is larger and heavier than the male. The average weight of the female is 2.3 kg compared to 1.8 kg for the male. Adult males may be almost pure white in colour. Adult females are darker, their white feathers barred with dark brown. First-year birds of both sexes are more darkly marked than their adult counterparts. Immature males resemble adult females, and immature females are heavily barred and may appear dark grey when seen from a distance. The light coloration of Snowy Owls provides camouflage when the owls are perched on snow, but this advantage is lost in summer. As spring approaches and the ground becomes bare, Snowy Owls move to sit on patches of snow or ice. No one knows whether they do this to camouflage themselves or whether they are merely keeping insects away or staying cool. A dense layer of down, overlaid with thick feathering, insulates the Snowy Owl’s entire body, including the legs and toes, and enables the bird to maintain a body temperature of 38 to 40°C, even when the air temperature reaches -50°C. In strong wind, Snowy Owls may seek shelter by crouching on the ground behind a windbreak, such as a pile of stones, snowdrift, or bale of hay. The ear-like feather tufts characteristic of many species of owls are greatly reduced in Snowy Owls and are rarely visible, giving the head a typically rounded outline. The bill is black and almost hidden by surrounding feathers. The yellow eyes are surrounded by disks of stiff feathers that reflect sound waves to the ear openings located immediately behind. The Snowy Owl’s acute hearing helps the bird to detect prey in dim light, when vision is limited. 1 Snowy owl in flight Snowy Owl The powerful feet are equipped with curved, black claws 25 to 35 mm long, and can quickly subdue even the largest prey. Signs and sounds Snowy Owls are rather shy and usually silent, unless nesting. They will hiss, scream, or snap their bill at those intruding on their territories, and will dive at, or even strike, human intruders at their nests. During the breeding season, the male advertises his presence on the territory with loud hooting and will attack any intruding male. Habitats and Habits In the breeding season, Snowy Owls are typically found from the treeline to the northern limit of Canada, preferring high, rolling tundra with tall points of land for nest sites and perches. In the High Arctic, nests are typically situated among such upland plants as willow, saxifrage, heather, and lichens, but in the Low Arctic, Snowy Owls tend to use dense, hummocky, dwarf shrub meadow for nesting. During winter in southern Canada, Snowy Owls inhabit prairies, marshes, open fields, or shorelines, habitats that resemble the treeless tundra of their breeding range. Although some individuals may wander in winter, many establish and defend hunting territories for periods of two or three months. Snowy Owls spend much of their time perched on fence posts, haystacks, trees, buildings, utility poles, or other sites where the view is unrestricted. They constantly scan the area around their perches, ready to chase another owl from the territory or to launch a silent attack on a mouse or other prey. Unique characteristics The eyes of owls do not move in their sockets. To look to the side or to follow a moving object, the bird must swivel its head. The eyes contain many more light-gathering cells than do human eyes and can detect small objects moving at great distances. As in other owls, the visual fields of the two forward-directed eyes overlap widely. This binocular vision gives owls excellent ability to judge distances, an important attribute when attacking prey. Unlike most owls, which are nocturnal, or active only at night, the Snowy Owl is active during the day and at dawn and dusk. Because daylight is continuous within the Arctic Circle during much of the summer nesting season, this adaptation is not surprising. In 1987, the Snowy Owl was named the official bird of Quebec. 2 Snowy Owl Range Snowy Owls breed on the arctic tundra of both Eurasia and North America. The Canadian breeding range includes the islands of the Arctic Archipelago, from Ellesmere Island in the north, to Baffin Island in the east, to Banks Island in the west, and along the northern coast of the continent from Yukon Territory to Labrador. In winter the Snowy Owl visits many parts of southern Canada, providing most Canadians with the opportunity to observe one of the most striking and distinctive of the world’s more than 200 species of owls. Some Snowy Owls remain over the winter in the areas where they nest. Snowy Owls have been seen in midwinter as far north as 82° on Ellesmere Island, where darkness is continuous at this season. Snowy Owls also migrate to more southerly latitudes, usually beginning in November. In certain parts of their wintering range (on the prairies of western Canada and in the unforested parts of southern Ontario and Quebec, as well as in adjacent regions of the northern United States), they are regular visitors, although their numbers vary from year to year. 3 Snowy Owl In other wintering areas, along the Pacific coast of Canada and the northern United States and in the Atlantic provinces and New England, their occurrence is less regular. In these regions, a winter with a large number of Snowy Owls may be followed by several years in which none are seen. It is mostly first-year birds that visit these less frequented areas, with relatively few adult owls appearing. Individual Snowy Owls have been recorded as far south as central California, Texas, and Georgia, but such records are exceptional. It was once believed that Snowy Owl migrations were periodic, occurring regularly every three or four years, and that they matched the population lows of arctic lemmings. However, recent analyses of Christmas Bird Counts show that the numbers of Snowy Owls wintering in various parts of North America fluctuate irregularly from year to year. Some Snowy Owls may also migrate between Russia and Canada. Feeding Although fast enough to capture ducks on the wing, the Snowy Owl prefers small mammals as prey. In the Arctic it may eat arctic hares, ptarmigan, foxes, or seabirds when available, but lemmings are its primary prey. These rodents, resembling large meadow mice, are very heavy breeders and their populations grow rapidly, eventually outstripping the food supply. At such times, starvation, disease, and predation cause lemming numbers to dwindle rapidly until the lemmings seem on the point of vanishing. From this low, the population gradually recovers until, three or four years later, it again reaches a peak. These population fluctuations of lemmings are consistent over areas of the tundra as large as 2 500 km2, with important consequences for the breeding biology of Snowy Owls in these areas. On their winter range, Snowy Owls also feed mainly on small rodents, usually meadow voles and white-footed or deer mice. Snowy Owls that winter near grain elevators or garbage dumps may feed almost exclusively on rats. However, Snowy Owls will hunt for what is available and will feed on mammals ranging in size from shrews to jackrabbits and on birds ranging from sparrows to ducks and pheasants. Prey are usually captured at the end of a short flight from a perch, although Snowy Owls also hunt on the wing, especially on the flat arctic tundra, flying slowly 10 to 15 m above the ground, ready to drop on any prey. Snowy Owls, like other birds of prey, swallow small prey items whole. Strong stomach juices digest the flesh, and then the indigestible bones, teeth, fur, and feathers are compacted into oval pellets that the bird regurgitates, or brings up, 18 to 24 hours after feeding. Regurgitation often takes place at regular perches where dozens of pellets may be found. Biologists frequently examine these pellets to determine the quantity and types of prey eaten. In southern Canada, the pellets most commonly contain the fur and bones of meadow voles and other mice. Each Snowy Owl must capture the equivalent of 7 to 12 mice a day to meet its food requirements. Breeding Snowy Owls that winter in southern Canada and the northern United States begin moving northward to their arctic breeding grounds in February and March. Snowy Owls sometimes gather in pairs or small groups at this time, and occasionally as many as 20 Snowy Owls may be seen perched within a few hundred metres of one another. Most are gone from their wintering range by April. 4 Snowy Owl Migrating Snowy Owls return to their breeding grounds while the tundra is still snow-covered. Each pair occupies a breeding territory of 1 to 2 km2 in area. Courtship begins in early May. The male performs display flights with exaggerated wing beats, as well as ground displays in which he stands erect in front of the female with his wings partially spread.
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